It is possible to identify particular sites within the myocardium which may benefit from local drug release therapy. Examples of problematic tissue which may benefit form local drug release therapy are ischemic sites and arrhythmogenic sites. Different means and methods for delivering agents to these sites will be disclosed in detail.
Ischemic Sites
Ischemic tissue is characterized by limited metabolic processes which cause poor functionality. The tissue lacks oxygen, nutrients, and means for disposing of wastes. This hinders the normal functioning of the heart cells or myocytes in an ischemic region. If an ischemic, or damaged, region of the heart does not receive enough nutrients to sustain the myocytes they are said to die, and the tissue is said to become infarcted. Ischemia is reversible, such that cells may return to normal function once they receive the proper nutrients. Infarction is irreversible.
Non-invasive systemic delivery of anti-ischemic agents such as nitrates or vasodilators allows the heart to work less by reducing vascular resistance. Some vascular obstructions are treated by the systemic delivery of pharmacological agents such as TPA, urokinase, or antithrombolytics which can break up the obstruction. Catheter based techniques to remove the vascular obstructions such as percutaneous transluminal coronary angioplasty (PTCA), atherectomy devices, and stents can increase myocardial perfusion. More drastic, but very reliable procedures such as coronary artery bypass surgery can also be performed. All of these techniques treat the root cause of poor perfusion.
It should be noted that these therapies are primarily for the treatment of large vessel disease, and that many patients suffer from poor perfusion within smaller vessels that cannot be treated with conventional therapies.
The delivery of angiogenic growth factors to the heart via the coronary arteries by catheter techniques, or by implantable controlled release matrices, can create new capillary vascular growth within the myocardium. Recent work has shown substantial increases in muscular flow in a variety of in vivo experimental models with growth factors such as Tumor Angiogenic factor (TAF), basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), and acidic fibroblast growth factor (aFGF). The methods of delivering these agents to the heart include implantable controlled release matrices such as ethylene vinyl acetate copolymer (EVAC), and sequential bolus delivery into the coronary arteries. Recently similar techniques have been attempted in peripheral vessels in human patients with the primary difficulty being systemic effects of the agents delivered.
U.S. Pat. No. 5,244,460 issued to Unger describes a method of introducing growth factors over time by delivering them through fluid catheters into the coronary arteries, but this does not result in efficient delivery of these agents to the ischemic tissue. If these or other agents are delivered to the coronary artery, a region of tissue that is equivalent to that supplied by the artery will receive the therapeutic agents. This may be substantially more tissue than is in need of local drug delivery therapy. Further, if a vessel is occluded, the growth factors will act in the tissue which the coronary arteries successfully perfuse. As the underlying problem of ischemic tissue is poor perfusion, excess growth factor must be delivered in order to obtain the desired effects in the poorly perfused tissue. Further, growth factors may cause unwanted angiogenesis in tissues where inappropriately delivered. The cornea is described by Unger as such a location, but perhaps more critical is inappropriate delivery of these factors to the brain. Further, placement of delivery devices within these coronary arteries as Unger describes tends to obstruct these arteries and may augment occlusive thrombosis formation. There is a significant need for minimizing the amount of growth factors for introducing angiogenesis by delivering these agents only to the site where they are most needed.
There are complications with clinically acceptable procedures where special devices for delivering agents to ischemic tissue will be useful. After opening vessels using PTCA, the vessels often lose patentcy over time. This loss of patentcy due to re-stenosis may be reduced by appropriate pharmacological therapy in the region of the artery. There is a need for new techniques that will enable pharmacological therapy to reduce the incidence of restenosis.
Arrhythmogenic Sites
Cardiac arrhythmias are abnormal rhythmic contractions of the myocardial muscle, often introduced by electrical abnormalities, or irregularities in the heart tissue, and not necessarily from ischemic tissue. In a cardiac ablation procedure, the arrhythmogenic region is isolated or the inappropriate pathway is disrupted by destroying the cells in the regions of interest. Using catheter techniques to gain venous and arterial access to the chambers of the heart, and possibly trans septal techniques, necrotic regions can be generated by destroying the tissue locally. These necrotic regions effectively introduce electrical barriers to problematic conduction pathways.
U.S. Pat. No. 5,385,148 issued to Lesh describes a cardiac imaging and ablation catheter in which a helical needle may be used to deliver fluid ablative agents, such as ethanol, at a depth within the tissue to achieve ablation. Lesh further describes a method of delivering a pharmacological agent to the tissue just before performing the chemical ablation procedure to temporarily alter the conduction of the tissue prior to performing the ablation. Such temporary alteration of tissue has the advantage of allowing the physician to evaluate the results of destructive ablation in that region prior to actually performing the ablation. This method of ablation has the advantage that the ablative fluid agents are delivered to essentially the same tissue as the temporary modifying agents. However, with ablative fluid agents it is difficult to control the amount of tissue which is destroyed—especially in a beating heart, and ablative RF energy is in common use because of its reproducible lesions and ease of control. There is a need for an ablation catheter that uses a single structure within the heart wall for both temporary modification of tissue conductivity by delivery of therapeutic agents at a depth within the tissue, and delivery of RF energy.
U.S. Pat. No. 5,527,344 issued to Arzbaecher and incorporated by reference herein, describes a pharmacological atrial defibrillator and method for automatically delivering a defibrillating drug into the bloodstream of a patient upon detection of the onset of atrial arrhythmias in order to terminate the atrial arrhythmias. By delivering agents to a blood vessel, Arzbaecher requires systemic effects to be achieved in order to terminate the atrial arrhythmias. The advantages of local drug delivery are absent from the system described. There is a need for a system and method to transiently treat atrial arrhythmias by local delivery of pharmacological agents which affect the excitation of the cardiac tissue locally.
Many patents describe systems for delivering anti inflammatory agents to the endocardial surface of the heart. Such surface delivery is less viable for regions at a depth within the tissue. Further, because of the volume of fluid moving by the inner surfaces of the heart, higher concentrations may be required at the surface to counteract the effects of dilution. These higher doses result in greater likelihood of problematic systemic effects from the therapeutic agents. Delivering agents within the tissue will minimize the dilution of agents, and decrease the possibility of the agents being delivered to inappropriate sites. This is particularly important with growth factors whose systemic affects are not well documented, just as it is important for antiarrhythmic agents whose pro-arrhythmia systemic effects have been recognized. There is a need for a means to deliver agents to ischemic and arrhythmogenic sites within the myocardium.
To deliver substances at a depth within the heart, U.S. Pat. Nos. 5,447,533 and 5,531,780 issued to Vachon describe pacing leads having a stylet introduced anti inflammatory drug delivery dart and needle advanceable from the distal tip of the electrode. U.S. Pat. No. 5,002,067 issued to Berthelson describes a helical fixation device with a groove to provide a path to introduce anti-inflammatory drug to a depth within the tissue. U.S. Pat. No. 5,324,325 issued to Moaddeb describes a myocardial steroid releasing lead whose tip of the rigid helix has an axial bore filled with a therapeutic medication such as a steroid or steroid based drug. None of these patents provides a means for site specific delivery of agents as all applications of the drug delivery systems are at the location selected for pacing. None of these provides a means or method for delivering agents to ischemic or infarcted tissues. Only Vachon and Moaddeb provide a means for effectively delivering the anti-inflammatory agents to a depth within the myocardium. U.S. Pat. No. 5,551,427 issued to Altman describes a catheter system capable of delivering drugs to the heart at a depth within the heart tissue.
U.S. Pat. No. 5,431,649 issued to Mulier describes a hollow helical delivery needle to infuse the heart tissue with a conductive fluid prior to ablation to control the lesion size produced. The system does not have drug delivery capabilities.
None of the prior art includes the use of macromolecular controlled release matrices such as ethylene vinyl acetate copolymer to deliver agents with large molecular weights to a depth within the heart tissue.